Step up to the HD580/600 and you'll probably be much happier long-term, as they're much more sonically refined than the lower-tier Sennheiser models. You can pick up a used pair of HD580 (which I own now) for under $150 and the HD600 (which I used to own and should have kept: they're definitely a touch more transparent) for around or just above $200.
For context, I owned the HD555 modded to HD595 (model prior to the HD598), and the HD580/600 series is a huge step up. The lower models have a mid-bass bump and some grainy treble emphasis, but the 580/600 sounds smooth and neutral top to bottom (though some find them too warm).
Here's a great recent comparison of the HD580/600/650 series:
http://www.innerfidelity.com/content/very-important-sennheiser-hd-580-hd-600-and-hd-650
For context, I owned the HD555 modded to HD595 (model prior to the HD598), and the HD580/600 series is a huge step up. The lower models have a mid-bass bump and some grainy treble emphasis, but the 580/600 sounds smooth and neutral top to bottom (though some find them too warm).
Here's a great recent comparison of the HD580/600/650 series:
http://www.innerfidelity.com/content/very-important-sennheiser-hd-580-hd-600-and-hd-650